A flail is a kind of weapon. A samurai will know it as a nunchaku.
A flail does decent damage and is relatively light, but the lack of any artifact flails makes it fairly worthless to invest skill points in. Priests are perhaps the only role that would benefit from training in flail.

Origin

The word flail has been used somewhat loosely to describe any blunt weapon with one or more heads that are flexibly attached to a handle by a chain, rope, hinge, et cetera. While it may seem jarring to players familiar with Dungeons & Dragons weaponry to group nunchaku with the spherical-headed flails seen in media, the term has historically been used for both cylindrical- and spherical-headed weapons.

Cylindrical-headed flails were two-handed weapons adapted from agricultural tools (used to remove the husks from grain by beating) and commonly used by infantry (perhaps including farmers who had to supply their own weapons). Compare the origins of many polearms. The Japanese nunchaku possibly had a similar origin.

One-handed flails with spiked metal heads on chains (sometimes called a "mace-and-chain"), as seen in fantasy media, appeared in late medieval art, but it is unclear how common they actually were as weapons. The chain would have made the head(s) difficult to control, but might have allowed the wielder to hit targets behind shields.

Variants

In the Convict Patch and variants that include the Convict role, a wielded heavy iron ball functions as a melee weapon that uses the flail skill. In some versions of the role, using the ball in combat even trains the skill.